Lamination is carried out in a post film formation step. In order for lamination to be feasible, the moisture vapor permeable film must have enough structure, tensile strength and tear strength such that the film can be formed, wound onto a roll, and later unwound and handled during the lamination process. The lamination can involve one or more methods; thermal lamination (melting an existing component), adhesive lamination (adding a liquid component just prior to laminating), ultrasonic lamination (a vibration process which softens or melts components, similar to thermal lamination), etc. It is extremely difficult to handle moisture vapor permeable films less than 20 microns (0.8 mil) in thickness during the adhesive lamination process without introducing holes into the film. Thus, when adhesive lamination has been used to attempt to make composite sheets with thinner films, the composite sheets have not exhibited the fluid barrier properties (e.g., hydrostatic head, dynamic fluid transmission) desirable for a composite sheet designed for use in absorbent articles or medical apparel.
Thermal lamination of moisture vapor permeable films less than 20 microns thick has similarly resulted in composite sheet materials with inadequate barrier properties. When composite sheets are made by thermally laminating a thin film to a fibrous substrate, the thin film handling problems associated with adhesive lamination as described above are encountered. In addition, in order to carry out a thermal lamination, the film must be subjected to elevated temperatures and pressures so as to soften the film and force it into mechanical engagement with the fibrous substrate. Generally, the peel strength between the film and the fibrous substrate increases with increasing extrusion melt temperatures and increasing nip pressures. Unfortunately, when moisture vapor permeable films with a thickness of less than about 20 microns are subjected to the elevated temperatures and pressures needed to obtain adequate peel strength in the composite sheet, small holes develop in the film such that the composite sheet does not exhibit the fluid barrier properties desired in a composite sheet for use in absorbent articles or medical apparel. These small holes can result from the non-uniform temperature throughout the web during bonding and from high bonding pressures used in the prior art.
Extrusion coating processes disclosed in the prior art are similarly unable to generate a composite sheet with a thin moisture vapor permeable film of less than about 20 microns (0.8 mil) that also has the barrier properties and moisture vapor transmission properties desirable for use in medical apparel and absorbent article applications. In an extrusion coating process, the polymer that forms the film is melted at an elevated temperature to reduce its viscosity such that when the polymer melt is coated onto the fibrous substrate and passed through a nip, the melt is pressed into engagement with the fibrous network of the substrate. Unfortunately, the low viscosity of the melted polymer, the pressure of the nip, and the thinness of the film each contribute to the generation of small holes in the film. In addition, thinner films are more susceptible to fiber protrusion through the film, which also contributes to small holes.
Accordingly, there is a need for a composite sheet material that acts as a barrier to fluids, yet is also highly permeable to moisture vapor, with improved mechanical properties, such as enhanced tensile strength, tear resistance and puncture resistance.
Existing membranes, for example, those designed to be located on roofs under terra-cotta tiles, have certain limitations. For example, some are heavier than about 170 grams/square meter, but have good tear resistance. The materials of the present invention may have weights as low as about 100 grams/square meter, while maintaining good tear resistance, or at about 200 grams/square meter, having very high tear resistances. Typically these membranes use laminates containing microporous polyolefin films. These films have intrinsic breathability variability due to the mechanical nature of the porosity (microscopic tears or holes that can fail to form in the extrusion process, or may “heal” in the lamination process).